The present invention relates to a method to control and improve the structure of continuously cast metallic products during their solidification.
It is already known to subject metals during their solidification, and while being continuously cast, to the action of a magnetic field in order to cause movement of the liquid metal in the interior of the continuously cast product. The magnetic field, which is usually created by an inductor arranged in close proximity to the outer surface of the continuously cast product, will create in the product currents and consequently mechanical forces which will produce in the liquid non-solidified portion of the continuously cast product a movement of the liquid metal.
It is also known that by such movement of the liquid metal a modification of the structure of solidification of the cast product will be obtained. In fact, such structure will depend on various parameters such as the temperature gradient in the zone of the beginning solidification, the speed of growth of the solid portion of the cast product, the concentration of the solute in the liquid, and these parameters are modified by imparting a movement to the liquid metal in the center of the cast product. These various factors are interdependent and it is only possible by experimentation to determine the overall qualitative influence on the structure of the solidified product, obtained by imparting a movement to the liquid metal during its solidification.
The general aim of imparting by an electromagnetic field, a movement of the liquid metal during its solidification, is to avoid the formation of a basaltic structure in the center of the cast product. The term basaltic structure is to be understood as a structure resulting from a solidification in the form of branched dendrites oriented according to the temperature gradient, that is generally perpendicular to the outer surface of the cast product. This type of solidification is particularly encountered during the casting of steel and it occurs usually in a portion of the product in a zone intermediate the outer surface and the core of the product and this zone is liable to extend at least locally up to the center of the product. It has been ascertained that this type of solidification may impart to the product unfavorable mechanical characteristics even after treatment of the same by subsequent forging or hammering.
The favorable effect of an electromagnetic movement or mixing of the liquid metal to suppress such basaltic growth and to replace it by a solidified structure with an improved quality is well known. Particularly, it has already been proposed to utilize this technique to act on the structure of solidification of products which are continuously cast. In fact, the transformation of such products into products of improved quality will be obtained rather by a relatively slow rate of casting than in the case of big ingots obtained by casting the same in the usual manner, which does not permit to mitigate with sufficient efficiency formation of a structure of solidification of the basaltic type.
However, experimental observations have so far not permitted to apply the electromagnetic mixing technique in such a manner so as to obtain a predetermined desired result, regardless of the type of product cast and the conditions during such casting. In the technical exploitation of this technique it is, however, important to impart to the cast product predetermined characteristics which are relatively constant.